The myosin essential light chain (ELC) is a structural component of the actomyosin cross-bridge, but little is known about its function or the significance of the cardiac specific N-terminal ELC extension on the interaction of myosin with actin, development of force and heart performance. The functional importance of ELC is highlighted by the recent identification of several missense mutations shown to cause Familial Hypertrophic Cardiomyopathy (FHC), a genetic disorder manifested by ventricular enlargement, myofilament disarray and sudden cardiac death (SCD). This proposal will elucidate the mechanisms of ELC-mediated FHC and the role of the direct N-terminal ELC-actin interaction in the normal and FHC heart. SPECIFIC AIM 1: DETERMINE THE MECHANISM FOR THE ELC-MUTATION INDUCED DEVELOPMENT OF FHC. We hypothesize that ELC mutations may lead to FHC by affecting the ability of the ELC to form stable structures with the MHC, thus altering the mechanical (stiffness, force) and/or kinetic properties of the myosin cross-bridge during the contractile cycle and development of force. This study is focused on two FHC- mutations localized in two different ELC domains and demonstrating different clinical phenotypes. The N- terminal A57G mutation was shown to cause a classic asymmetric hypertrophy and SCD, and the C-terminal E143K mutation, which presented with mid-cavity hypertrophy, obstruction in systole and restrictive physiology. We propose to determine: (1A) The effect of FHC mutations on the binding of ELC to the 2-MHC, myosin lever arm stiffness and myosin cross-bridge kinetics. (1B) The contractile properties of the mutated myocardium in skinned and intact papillary muscle fibers. (1C) The age-dependent morphology and function of mutated transgenic mouse hearts by histopathology, echocardiography, invasive hemodynamics and MRI. SPECIFIC AIM 2: EXPLORE THE MECHANISMS BY WHICH THE N-TERMINUS OF ELC REGULATES MYOSIN FORCE GENERATING CAPABILITY AND CARDIAC MUSCLE CONTRACTION. We hypothesize that the N- terminus of ELC interacts with the actin-tropomyosin-troponin filaments to promote strong cross-bridge formation~ thus it serves as a mechanism to modulate the actin-myosin interaction and myofilament cooperativity, thereby regulating contractile force. Using cardiac muscle preparations from Tg- 43 mice expressing the N-terminal truncated ELC and Tg-WT expressing the full length ELC wild-type, we will study: (2A) the importance of the N-terminus of ELC for myosin force generating capability determined at the molecular and muscle fiber levels. (2B) the effect of the N-terminal ELC extension on myosin cross-bridge kinetics.(2C) The mechanism of the N-terminus ELC-dependent development of cardiac hypertrophy. These studies will help to determine the role(s) of the ELC and the N-terminal cardiac specific ELC extension in the regulation of contractile force in healthy and FHC-diseased hearts.